The present invention relates to a keyboard construction and, more particularly to a flat panel keyboard unit for an electronic apparatus using a pressure sensitive conductive material as contacts.
Conventionally, touch-keyboard units as shown in FIGS. 1A, 1B and 2 are proposed for an electronic apparatus such as an electronic calculator and the like. For example, the keyboard unit of FIGS. 1A, and 1B is disclosed in U.S. Pat. No. 3,911,234, issued on Oct. 7, 1975, by Yasumasa KOTAKA, entitled "KEYBOARD TYPE SWITCH ASSEMBLY HAVING FIXED AND MOVABLE CONTACTS DISPOSED ON FOLDABLE FLEXIBLE PRINTED CIRCUIT BOARD". The keyboard unit of FIGS. 1A, and 1B comprises a flexible ornamental plate 11 made of a polyester film and the like, printed key symbols thereon, a flexible base 12 having a key switching wiring pattern thereon, a spacer 13 provided with apertures (13-1) corresponding to the positions of each of the electrodes of the wiring pattern, and a support plate 14.
The flexible base 12 is folded, so that an upper electrode pattern and a lower electrode pattern face each other. The spacer 13 is inserted between the upper electrode pattern 12-1 and the lower electrode pattern 12-2. The layers are adhered to each other by the adhesive 15. 11-1 designates a key symbol printed section and 12-3 designates an electrode drawing portion. The combination of the flexible base 12 and the spacer 13 is sandwiched between the ornamental plate 11 and the support plate 14, so that the flexible base 12, the spacer 13, the support plate 14 are adhered by the adhesive 15.
On the other hand, the keyboard unit of FIG. 2 comprises an ornamental plate 21 formed with key contacts on the back surface of the ornamental plate 21 by a conductive ink, a hard base 26 formed with a key wiring parttern on one surface of the hard base 26, and a spacer 23. 27 designates an LSI and 28 designates a cabinet. The ornamental plate 21, the spacer 23, and the hard base 26 are stacked and adhered together.
In the above conventional keyboard units as shown in FIGS. 1(1), 1(2), and 2, required key operating pressure is great due to the spacer and the like. If the keyboard unit is operated for a long time, the user's fingers become very tired. To reduce the above problems, the spacer may be made thinner and a diameter of each aperture of the spacer made larger. Keys may then close, however, due to a little vibration, so that misoperation of the key inputs occurs.
Particularly, in the keyboard unit of FIG. 2, the key electrodes become shorted by the possible distortion of the ornamental plate 21 printed with the key symbols thereon if the keyboard unit is used for a long period. Also, the outline of the spacer may appear through the surface of the ornamental plate 21 impairing the appearance of the apparatus. If these keyboard units are used in electronic apparatus such as an electronic calculator, it is difficult for the electronic calculator to be made very thin.